Tympanometry or immittance Audiometry is used to check the movement of the tympanic membrane. Tones are "bounced" off the tympanic membrane to determine the amount of sound that is reflected back through the ear canal. If the tympanic membrane is very stiff (eg. with middle ear effusion) then all the sound is reflected back. When the tympanic membrane moves normally, very little of the tone is reflected.
- Ear Canal volume: A larger than average ear canal volume is consistent with a perforation in the tympanic membrane or a patent grommet. A smaller than average ear canal volume is consistent with wax blockage, a collapsing canal or the probe being pushed against the ear canal wall. (See below for normative data).
- Middle ear pressure: In the normal ear the Eustachian tube ventilates the middle ear cavity maintaining the air pressure at or near atmospheric pressure. If the Eustachian tube is blocked, the air in the middle ear is absorbed by the blood vessels of the mucosal lining and negative air pressure develops. This results in the tympanic membrane becoming retracted and if the Eustachian tube remains blocked, fluid may build up in the middle ear. If fluid is present, then the tympanic membrane is not moving, and thus is permanently "locked" in place.
- Compliance (mobility): This looks at how mobile the tympanic membrane is. If the eardrum is more mobile than average it can indicate a thin tympanic membrane or ossicular discontinuity. When the tympanic membrane is stiffer than average it can be due to scaring on the eardrum, or to a fixed ossicular chain.
- Contraindications for performing immittance audiometry
- Recent stapedectomy or other middle ear surgery
- Discharging ear
- Discomfort (eg severe otitis externa, herpes zoster oticus)
- Tympanometry norms:
- Peak in tympanogram between + or - 100 daPa.
- ASHA (1990).
- Children have a compliance measure between 0.2 and 0.9 ml inclusive.
- Adults have a compliance measure between 0.3 and 1.4 ml inclusive.
- Ad is a peak with static admittance greater than these values.
- As is a peak with static admittance lesser than these values.
Type B low
- No identifiable peak. Equivalent ear canal volume within normal limits.
- Normal values Mean 90% range
- Children (Margolis & Shanks, 1985) 0.7 0.4 - 1.0
- Adults (ASHA, 1990) 1.1 0.6 - 1.5
Type B high
No identifiable peak. Equivalent ear canal volume exceeds normal limits.
Peak in the tympanogram at greater than -100 daPa. This can be shallow or deep as well.
An acoustic reflex looks at the middle ear, inner ear, auditory nerve, brainstem nuclei and facial nerve. An abnormal acoustic reflex can be indicative of a number of pathologies, and because of this the acoustic reflex is examined in conjunction with Tympanometry and pure tone audiometry to better isolate the part of the auditory pathway that is not functioning as well as expected.
It is important to remember that neither Tympanometry nor Acoustic Reflex tests assess hearing. A person with a normal tympanogram can still have a hearing loss. It is also important to realise that a more permanent sensorineural loss can exist with a middle ear problem such as glue ear. The only way of making sure that a child or adults hearing is normal is to have them tested by an audiologist.